1. Field of the Invention
Embodiments of the present invention generally relate to determination of a characteristic wavelength of an optical component and, more particularly, to techniques and apparatus for controlling the manner in which a spectral bandwidth is swept in an effort to determine the characteristic wavelength.
2. Description of the Related Art
Many optical components have a characteristic wavelength that may be found by interrogating the optical component with an optical source capable of producing light at various wavelengths over a fixed range or bandwidth. For example, Bragg gratings (typically formed by photo-induced periodic modulation of the refractive index of an optical waveguide core) are highly reflective to light having wavelengths within a narrow bandwidth centered at a wavelength generally referred to as the Bragg wavelength. Because light having wavelengths outside this narrow bandwidth is passed without reflection, Bragg wavelengths can be determined by interrogating a Bragg grating with a light source swept across a bandwidth that includes the Bragg wavelength and monitoring the reflected optical power spectrum at a receiver unit. Because Bragg wavelengths are dependent on physical parameters, such as temperature and strain, Bragg gratings can be utilized in optical sensor systems to measure such parameters.
In these and a wide range of other types of optical systems, the measurement of a characteristic wavelength of an optical component to great accuracy (and/or with great repeatability) is important to system performance. Two significant parameters determining the error of any such measurement are the signal to noise ratio (SNR) and effective interrogation time of the measuring system. SNR is dependent of many factors including received optical power, optical-source noise, and receiver noise. The effective interrogation time is dependent on overall averaging time and the proportion of that time which is producing useful signals at the receiver unit. Improving these two parameters can improve characteristic wavelength measurement repeatability and accuracy.
In a typical system, with a fixed spectral bandwidth sweep, a large percentage of the interrogation time is spent covering wavelengths where no useful signal is returned by the optical element under test. This may be particularly true in the case where multiple elements (e.g., multiple Bragg gratings disposed serially on a common fiber) are combined in a commonly used wavelength-division multiplexing (WDM) scheme. In these arrangements, wavelength guard-bands are typically required between the spectral features of elements, for example, to ensure the elements have non-overlapping spectral features over the entire expected measurement range and even as some movement in the spectral features may be expected over time. These guard-bands increase the total range of wavelengths scanned, thereby increasing the amount of interrogation time spent covering wavelengths that produce no useful signal.
Accordingly, techniques and systems that optimize the useful received signal, reduce SNR, and reduce the total amount of interrogation time would be desirable.